Asphalt

Asphalt is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits. There is some disagreement amongst chemists regarding the structure of asphalt, but it is most commonly modeled as a colloid, with asphaltenes as the dispersed phase and maltenes as the continuous phase. In U.S. terminology, asphalt (or asphalt cement) is the carefully refined residue from the distillation process of selected crude oils. Outside of North America, the product is called bitumen.

The primary use of asphalt (bitumen) is in road construction, where it is used as the glue or binder for the aggregate particles. The road surfacing material is usually called 'asphalt concrete' in North America or simply 'asphalt' elsewhere. The apparent interchangeability of the words 'asphalt' and 'bitumen' causes a lot of confusion outside of the road construction industry despite quite clear definitions within industry circles.

Background

Asphalt or bitumen can sometimes be confused with tar, which is a similar black thermo-plastic material produced by the destructive distillation of coal. During the early and mid twentieth century when town gas was produced, tar was a readily available product and extensively used as the binder for road aggregates. The addition of tar to Macadam roads lead to the word Tarmac which is now used in common parlance to refer to road making materials. However, since the 1970s, when natural gas succeeded town gas, asphalt (bitumen) has completely overtaken the use of tar in these applications.

Asphalt can be separated from the other components in crude oil (such as naphtha, gasoline and diesel) by the process of fractional distillation, usually under vacuum conditions. A better separation can be achieved by further processing of the heavier fractions of the crude oil in a de-asphalting unit, which uses either propane or butane in a supercritical phase to dissolve the lighter molecules which are then separated. Further processing is possible by "blowing" the product: namely reacting it with oxygen. This makes the product harder and more viscous.

Natural deposits of asphalt include Lake Asphalts (primarily from the Pitch Lake in Trinidad and Tobago and Bermudez Lake in Venezuela), Gilsonite, the Dead Sea between Israel & Jordan, and Tar Sands.

Asphalt is typically stored and transported at temperatures around 150 degrees Celsius (300 °F). Sometimes diesel oil or kerosene are mixed in before shipping to retain liquidity; upon delivery, these lighter materials are separated out of the mixture. This mixture is often called bitumen feedstock, or BFS. Some dump trucks route the hot engine exhaust through pipes in the dump body to keep the material warm. The backs of tippers carrying asphalt, as well as some handling equipment, are also commonly sprayed with diesel oil before filling to aid release.

Known uses:

Ancient times

In the ancient Middle East, natural asphalt deposits were used for mortar between bricks and stones, ship caulking, and waterproofing. The Persian word for asphalt is mumiya, which may be related to the English word mummy. Asphalt was also used by ancient Egyptians to embalm mummies.

In the ancient Far East, natural asphalt was slowly boiled to get rid of the higher fractions, leaving a material of higher molecular weight which is thermoplastic and when layered on objects, became quite hard upon cooling. This was used to cover scabbards and other objects that needed water-proofing. Statuettes of household deities were also cast with this type of material in Japan, and probably also in China.

Poured bitumen has also been used as a damp-proof course in building.

Rolled asphalt concrete

The largest use of asphalt is for making asphalt concrete for road surfaces and accounts for approximately 80% of the asphalt consumed in the United States. Roofing shingles account for most of the remaining asphalt consumption. Other uses include cattle sprays, fence post treatments, and waterproofing for fabrics.

Asphalt road surface is the most widely recycled material in the US, both by gross tonnage and by percentage. According to a report issued by the Federal Highway Administration and the United States Environmental Protection Agency, 80% of the asphalt from road surfaces' that is removed each year during widening and resurfacing projects is reused as part of new roads, roadbeds, shoulders and embankments.

Mastic asphalt

Mastic asphalt is a type of asphalt which differs from dense graded asphalt (asphalt concrete) in that it has a higher bitumen (binder) content, usually around 7-10% of the whole aggregate mix,as opposed to roller asphalt, which has only around 5% added bitumen. Another asphalt which is fast gaining global popularity is stone mastic asphalt (SMA). SMA's advantages over rolled asphalt is its high anti skid qualities due to its high aggregate density and the lack of void content (air pockets). Another advantage of SMA is its longer durability over alternative road asphalt surfaces, but its manufacture and application if not controlled closely, can result in slippery road surfaces due to excess bitumen pooling (bleeding) on to the surface.

Asphalt emulsion

A number of technologies allow asphalt to be mixed at much lower temperatures. These involve mixing the asphalt with petroleum solvents to form "cutbacks" with reduced melting point or mixtures with water to turn the asphalt into an emulsion. Asphalt emulsions contain up to 70% asphalt and typically less than 1.5% chemical additives. There are two main types of emulsions with different affinity for aggregates, cationic and anionic. Asphalt emulsions are used in a wide variety of applications. Chip Seal involves spraying the road surface with asphalt emulsion followed by a layer of crushed rock or gravel. Slurry Seal involves the creation of a mixture of asphalt emulsion and fine crushed aggregate that is spread on the surface of a road. Cold mixed asphalt can also be made from asphalt emulsion to create pavements similar to hot-mixed asphalt, several inches in depth and asphalt emulsions are also blended into recycled hot-mix asphalt to create low cost pavements.

Mixing with petroleum-contaminated soil

Sometimes asphalt can be mixed with the output from low-temperature thermal desorption.

Etymology

The word asphalt is derived from the late Middle English : from French asphalte, based on late Latin asphalton, asphaltum, from Greek asphalton, asphaltos (άσφαλτος).

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